In this project we have several aims: Aim 1: To integrate information obtained by functional biochemical studies on ectopically expressed non-structural HTLV-1 proteins to infectivity and persistence of HTLV-1 in relevant cellular models in vitro (dendritic cells and T-cells). Aim 2: To integrate information obtained by functional biochemical studies on ectopically expressed non-structural HTLV-1 proteins to infectivity and persistence of HTLV-1 in relevant animal models of HTLV-1 infection. The viral genome encodes mRNAs for several non-structural proteins that affect cellular pathways and modulate viral replication. One such protein, p12, encoded by orf I, localizes to the ER and Golgi and cellular membranes. The proteolytic cleavage of p12 dictates its cellular localization and functions. The removal of a non-canonical endoplasmic reticulum (ER) retention/retrieval signal within the amino terminus of p12 is necessary for trafficking to the Golgi apparatus and the generation of a completely cleaved 8 kDa protein. The 8 kDa protein traffics to the cell surface, is recruited to the immunological synapse following T-cell receptor (TCR) ligation and down-regulates TCR proximal signaling. The full length form of p12 resides in the ER and interacts with the beta and gamma-c chains of the interleukin-2 receptor (IL-2R), the heavy chain of the major histocompatibility complex (MHC) class I, as well as calreticulin and calnexin. Genetic analysis of ORF-I from ex vivo samples of HTLV-1-infected patients reveals frequent amino acid substitutions within orf-I that inhibit proteolytic cleavage, suggesting that ER associated functions of p12I may be selected in vivo. We plan to use reverse genetic methods and animal models to understand the contribution of orf-I variants to the maintenance of viral load in the host. The HTLV-1 orf II encodes p30II, a nuclear/nucleolar protein that not only regulates viral expression by a post-transcriptional mechanism, but also affects the expression of genes involved in host responses such as TLR-4. In addition orf II encodes p13, a small protein that we have recently demonstrate to decrease viral replication by targeting and degrading Tax, the viral transcriptional activator. We have recently demonstrated that p13 is a negative regulator of virus expression. Expressed separately, p13 localizes to the mitochondria, but in the presence of Tax, part of it is ubiquitinated, stabilized, and re-routed to the nuclear speckles. The p13 protein directly binds Tax, decreases Tax binding to the CBP/p300 transcriptional co-activator and, by reducing Tax transcriptional activity, suppresses viral expression. We have generated infectious HTLV-1 molecular clones that do not express p30 and/or p13. We are testing these viruses both in vitro and in vivo. We have found that p30 affects dramatically the ability of HTLV-1 to replicate in monocytoid-derived dendritic cells and is necessary for persistence of infection in macaques. Further studies are ongoing to determine the effect of p30 and p13 on in vivo infectivity is being tested in animal models.